Composting is well recognized as an environmentally sound and economical means of recycling a variety of organic materials, and is increasingly becoming an important part of municipal and industrial solid waste management programs. Various composting technologies are available or are being developed, including static pile composting, windrow composting, aerated-windrow composting, and in-vessel composting employing, for example, circular reactors, horizontal agitated bay or bin reactors and vertical reactors.
In all such systems there is need for cost effectiveness, including especially capital cost and operating cost effectiveness, and suitability for automation. Regarding capital costs, reducing the space required for a given throughput of composting material is a well recognized need in the industry. Composting operations employing windrows, for example, are thought to have a disadvantageously low ratio of composting material throughput to processing area square footage. While in vertical and closed reactors, compost material may typically be mounded 20-30 feet high, technical difficulties have been experienced involving the adequacy of aeration, leading in some cases to the composting process becoming anaerobic.
So called in-vessel composting operations, particularly those suitable for use in an enclosure building for sound and odor control, have been used with excellent results. A particularly advantageous system of this type, presently used and marketed by International Process Systems, Inc., a Wheelabrator Clean Water Company, and often referred to as the IPS Agitated Multi-Bay Composting System, employs automated agitators to thoroughly mix and aerate composting material in parallel bays. Starting at the discharge end of an open, elongate composting bay, the agitator moves through the bed of composting material toward the front, loading end of the bay. Typically, the agitator travels through each bay daily, mixing the material and rearwardly displacing it, i.e., moving it a given distance down the bay from the loading end toward the discharge end of the bay. Material typically is displaced a constant, fixed distance (often about 12 feet) down the bay with each pass, thus discharging finished material at the discharge end and creating space for a new load of organic material at the loading end with each pass. An agitator of this type and its use in an IPS agitated bay composting system is described in U.S. Pat. No. 4,828,399 to Pacentino et al, assigned to International Process Systems, Inc., that description being incorporated herein by reference.
Typically, the moisture content and the volatile content of composting material decreases during the composting operation. That is, as recognized by those skilled in the art, moisture loss and volatile solids content loss from the composting material during the composting process results in a reduction in volume of the composting material. Shrinkage in height of a uniform width "rick" of mushroom compost is said to be 30 to 40% in U.S. Pat. No. 3,776,528. While a somewhat randomly irregular bed height is shown in the illustrated portion of the composting bays in FIG. 18 of aforementioned U.S. Pat. No. 4,828,399, the beds would, in fact, have progressively less height from the tip-in end to the discharge end. This is a natural consequence of the composting material's loss of volume and the substantially uniform distance of rearward displacement of the composting material with each pass of the agitator through the bed of compost material in the bay.
In fact, in a typical IPS agitated bay composting system, solids content of the composting material starts at about 30%-50%, most often between 38% and 42%, and finishes at the discharge end of the bay with a solids content of about 50%-90% after an 18-24 day composting period. Consequently, the depth of the composting material bed in the bay may drop, for example, from about 5 feet at the receiving end of the bay to about 3 feet at the discharge end.
The open volume of the bay above the composting material represents an opportunity for improved efficiency of the system. The same amount of composting material could be contained in a shorter bay if the full design depth of the bay was utilized along its entire length. Shorter bays could be constructed at lower capital costs. In addition, they would require less land space or, alternatively, more bays could be placed within a given area. In addition, the agitator could process a larger number of bays, since it would pass through a shorter bay in less time. The same result is not achieved simply by loading additional material at the loading end of the bay, since material is loaded already to the full depth of the bay at that end. Also, the composting material may not simply be moved more quickly through the bay to fill the additional volume toward the discharge end, since residence time in the bay is dictated by the requirements of the composting process.
Various compost handling machines have added fixed or adjustable bed height limiters in an effort to produce a bed of more uniform height. Moveable side and top plates are suggested for forming so called stacking gates in the composting machine of U.S. Pat. No. 3,856,276 and U.S. Pat. No. 4,457,630, both issued to Pannell. Similarly, in aforesaid U.S. Pat. No. 3,776,528 to Toto, a composting machine has a trailing buffer which is adjusted to scrape over the rick of discharged material, pushing a wave of excess material before it. Such devices resist a compost bed exceeding a given height, but do not produce a bed which maintains a full bay depth. As noted above, in an in-vessel composting operation, use of the entire available design depth along the length of a bay could enable a given amount of material to be composted in a shorter bay without reduction in process retention time.